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Manifold vacuum:

A manifold vacuum is created when the engine is running, drawing the fuel-air mixture into the cylinders. 
Manifold pressure is measured in inches of mercury. An idling engine's manifold pressure reading of eighteen
inches of mercury is normal at sea level. At higher altitudes the reading would be lower. Manifold vacuum 
readings are a good indication of the engine's performance. Pressure readings should be taken at normal engine
operating temperature, directly from the intake manifold. With the engine running at idle, transmission in park 
or neutral (emergency brake set), take a reading of the gauge.
 
Here are indications of a troubled engine: low & steady, very low, needle fluctuates steadily as speed increases,
gradual drop in readings at idle speed, intermittent fluctuation, slow fluctuation or drifting of needle. 
Common causes of these indications are:

1. Low & Steady: Late ignition timing or valve timing, poor compression.

2. Very low: Leaking intake manifold, carburetor, and head gasket.

3. Needle fluctuates steadily as speed increases: Lose of power in a cylinder, leaking valve, head gasket,
    intake manifold, defective ignition, and weak valve springs.

4. Gradual drop in reading at idle speeds: Excessive exhaust system backpressure.

5. Intermittent fluctuation: Defective ignition, sticking valve.

6. Slow fluctuation or drifting of needle: Idle mixture adjustment, poor crankcase ventilation, and leaking 
    carburetor, intake manifold.